The present invention is in the field of printers and copiers. More specifically this invention relates to a receiver sheet supply and feed apparatus, including a vacuum corrugated feeder, and a positive air pressure separator on such printers and copiers. This invention is useful for the apparatus described by the U.S. Pat. No. 5,344,133 “Vacuum belt feeder having a positive air pressure separator and method of using a vacuum belt feeder” by Jantsch et al, which patent is hereby incorporated by reference in its entirety. The incorporated patent refers to a vacuum, a first positive air supply, and a second positive air supply. The first and second positive air supplies are used simultaneously and will herein be referred to collectively as the airknife.
In typical reproduction apparatus such as copiers or printers, information is reproduced on individual cut sheets of receiver material such as plain bond paper or transparencies. Such receiver sheets are stored in a stack and fed individually when copies are to be produced. The sheet feeder for the reproduction apparatus must be able to handle a wide range of sheet types and sizes reliably and without damage. Sheets must be fed individually, without misfeeds or multi-feeds.
In the vacuum corrugated belt feeder disclosed in the above patent, both the vacuum and the positive air pressure are controlled by valves. During the feed cycle, the positive air pressure valve is continuously open. The vacuum valve is opened to acquire the top sheet off the stack. After approximately 220 milliseconds (for a 110 pages per minute (ppm) feed rate), the clutch is actuated, which drives the feed belts to advance the sheet into the constantly rotating take away rollers. At a time after the lead edge of the sheet has reached the take away rollers, prior to the trail edge of the sheet reaching the edge of the ports in the vacuum plenum, the vacuum and the clutch are turned off.
The airknife airflow and velocity during the acquisition phase must be great enough to fluff the stack and pre-separate the top sheet. During the transport phase, the flow from the airknife must be high enough to create the air bearing between the sheet being fed, and the rest of the stack. However, flow that is too high during the transport phase has several undesirable effects. For example, if the flow is too high there is an increased tendency for the sheets below the top sheet to be blown back away from the lead edge. This is especially troublesome for sheets that do not have a continuous trail edge. Also, the air can deflect the lead edge of sheets with low stiffness, especially if the paper curl is down (lead edge away from the feed belts), which can lead to paper damage or jamming. The flow must not be so great as to levitate any sheets below the sheet being fed above the mechanical gate fingers along the lead edge of the paper drawer, or high enough to cause the second sheet to contact the top sheet when it is being transported off the stack. Also, if the flow is too great, it can cause the trail edge of the sheet being fed to flutter violently, which can in turn contact the sheet below it, tending to drive it forward also.
Typically, the minimum airflow of the airknife is dictated by the acquisition and separation needs and the maximum airflow of the airknife is limited by the transport phase. A method of operation is desired which will optimize the usefulness of the airknife during the acquisition and separation phase, while minimizing the detriments of the airknife during the transport phase.